An investigation of the zonal mean volume mixing ratios of water vapor and methane from January through May 1979, derived from the Nimbus 7 limb infrared monitor of the stratosphere (LIMS) and stratosphere and mesosphere sounder (SAMS) experiments is undertaken with the objective of inferring the water vapor mixing ratio of air that enters the stratosphere from the troposphere and, concomitantly, the yield of water vapor from photochemical oxidation of methane in the stratosphere. The results are necessarily imprecise owing to a high level of random error in the data (estimated to be roughly 15--20% for methane and 5% for water vapor) and possible systematic errors of unknown magnitude. However, the results are generally consistent with chemical/dynamical modeling by other workers. The water vapor yield averaged between 1.5 and 2.0, although variability with time and level was relatively large. Water vapor yield more consistent with model results was obtained when only nighttime LIMS data were used. The general indication is that the water vapor yield increases with height in the 16- to 30-mbar layer. It appears that the average water vapor mixing ratio of air entering the stratosphere during the period studied was ~3.25¿10-6. Possible implications of these results to the stratosphere-troposphere mass exchange problem are discussed. ¿ American Geophysical Union 1989